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      <h3>Detect a transparent proxy</h3>
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<p>
Many ISPs implement a transparent proxy for certain services (mostly http). Usually these proxies are not visible to the user so it is not possible determine if a connection problem is an issue with the remote host or the proxy. Tracing to ftp.cdrom.com on port 21 (ftp) returns a complete route to the server.
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    <PRE>$ tracetcp ftp.cdrom.com:ftp

Tracing route to 207.250.14.6 on port 21
Over a maximum of 30 hops.
1       2 ms    1 ms    1 ms    192.168.0.1
2       9 ms    10 ms   8 ms    10.152.79.254
3       10 ms   33 ms   11 ms   62.253.122.137  [rdng-t2cam1-b-v102.inet.ntl.com]
4       27 ms   13 ms   10 ms   62.253.121.129  [winn-t2core-b-ge-wan61.inet.ntl.com]
5       12 ms   10 ms   9 ms    62.253.184.117  [win-bb-b-so-320-0.inet.ntl.com]
6       10 ms   11 ms   13 ms   62.253.185.201  [pop-bb-a-so-000-0.inet.ntl.com]
7       11 ms   9 ms    10 ms   212.113.14.129  [pos7-0.hsipaccess2.london1.level3.net]
8       11 ms   12 ms   11 ms   212.187.131.125 [so-4-0-0.mp2.london1.level3.net]
9       78 ms   76 ms   78 ms   212.187.128.153 [so-1-0-0.bbr2.newyork1.level3.net]
10      76 ms   75 ms   77 ms   64.159.17.104   
11      85 ms   78 ms   93 ms   64.152.40.2     [us-nyc-c-r2-pos6-0.core.viatel.net]
12      78 ms   78 ms   81 ms   66.192.240.33
13      99 ms   97 ms   100 ms  168.215.53.41   [core-02-so-2-2-0-0.chcg.twtelecom.net]
14      123 ms  119 ms  118 ms  168.215.55.203  [dist-01-so-2-2-0-0.mpls.twtelecom.net]
15      119 ms  119 ms  119 ms  66.192.244.203  [hagg-01-ge-0-3-0-0.mpls.twtelecom.net]
16      119 ms  120 ms  120 ms  207.250.14.130
17      Destination Reached in 128 ms. Connection established to 207.250.14.6
Trace Complete.</PRE>
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<p>
Performing the same trace on port 80 (http) shows the the packets take quite a different route after hop 2 and arrive at the proxy at hop 5. Note that the proxy responds to with the same IP address as the requested host.
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    <PRE>$ tracetcp ftp.cdrom.com:http

Tracing route to 207.250.14.6 on port 80
Over a maximum of 30 hops.
1       1 ms    1 ms    1 ms    192.168.0.1
2       11 ms   11 ms   10 ms   10.152.79.254
3       10 ms   10 ms   9 ms    62.253.122.9   [rdng-t2cam1-a-v102.inet.ntl.com]
4       11 ms   11 ms   10 ms   62.253.121.1   [winn-t2core-a-ge-wan61.inet.ntl.com]
5       Destination Reached in 10 ms. Connection established to 207.250.14.6
Trace Complete.</PRE>
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